Hashbase exists, and in theory most Dat web apps could do a read-only version for legacy browsers, but since the tooling isn't there, you'd have to work out something by hand. The Dat ecosystem has pretty bad support for polyfills and bridging to the HTTP web right now. Many Dat demos I see today just fail hard over HTTP, maybe with a best-viewed-in message if the dev took some effort, but in principle it doesn't need to be that way.